The size of the headers will have an effect, but I found that the length played more of an effect. There are some calculators out there for doing this for the exhaust pulses, but I think that you will want to make sure that your primary tubes are around 32" long. The longer that you can make them, the more it will help on the torque.
I tried shortening the headers once on a 7800rpm 427 with 2 1/4" headers. I lost about 1/10th when I shortened them from 30" to 24".
Bill M